1. Technical Field
The present invention relates to a DC-DC converter (DC-DC converter device) that boosts or steps down the voltage of a DC power supply to supply the voltage to a load and, in particular, a DC-DC converter including a protecting function used when a short-circuit failure occurs in the circuit.
2. Related Art
For example, a DC-DC converter is mounted on an automobile as a power supply device to supply a DC voltage to various onboard devices or circuits. In general, a DC-DC converter has a voltage converter circuit (booster circuit or step-down circuit) including a switching element, a coil, a capacitor, and the like, and switches voltages of a DC power supply at a high speed to output a boosted or stepped-down DC voltage.
In the DC-DC converter, a large current flows in the circuit due to an abnormality such as a short circuit, and the large current may break down a circuit element. Thus, a protection circuit to prevent a circuit element from being broken down by a large current in an abnormal state is conventionally disposed. Japanese Unexamined Patent Publication No. 2005-51919, Japanese Unexamined Patent Publication No. 2006-14491, and Japanese Unexamined Patent Publication No. 2012-157191 (will be described later) show a power supply device including the protection circuit.
In Japanese Unexamined Patent Publication No. 2005-51919, an overvoltage protection FET (field effect transistor), a reverse connection protection FET, and a voltage detection circuit that detects a voltage of a DC power supply are disposed. When a voltage detected by the voltage detection circuit exceeds a predetermined value with a power supply switch being turned on, the overvoltage protection FET is turned off to prevent the circuit element of a power conversion circuit from being broken down. When the power supply switch is turned on with the DC power supply being reversely connected, the reverse connection protection FET is turned off to prevent the circuit element of the power conversion circuit from being broken down.
In Japanese Unexamined Patent Publication No. 2006-14491, a reverse connection protection FET that is turned on when a power supply is connected in the forward direction and turned off when the power supply is connected in the reverse direction is disposed on a power supply path, and a booster circuit that boosts an output from the FET is disposed. On the basis of the output from the booster circuit, the FET is turned on. Even though the power supply voltage is low, a stable output voltage can be supplied.
In Japanese Unexamined Patent Publication No. 2012-157191, in order to prevent an element from being broken down by a large current flowing when a load on an output side is short-circuited, an overcurrent protection function that restricts a current flowing in a switching element of a booster circuit on the basis of a first reference value and a short-circuit protection function that quickly restricts the current on the basis of a second reference value larger than the first reference value are included.